Semiconductor memory device

ABSTRACT

According to one embodiment, a semiconductor memory device includes: a first memory cell; a second memory cell; a first word line; a second word line; and a first bit line. The device is configured to execute a first operation, a second operation, and a third operation to write data into the first memory cell. In the first operation, a first voltage is applied to the second word line. In the second operation, after the first operation, a second voltage higher than the first voltage is applied to the second word line. In the third operation, after the second operation, a third voltage higher than the second voltage is applied to the first word line, and a fourth voltage lower than both the second voltage and the third voltage is applied to the second word line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 16/852,733, filed Apr. 20, 2020, which is a continuation applicationof U.S. application Ser. No. 16/298,135, filed Mar. 11, 2019 (now U.S.Pat. No. 10,672,478, issued Jun. 2, 2020), which is based upon andclaims the benefit of priority from Japanese Patent Application No.2018-175787, filed Sep. 20, 2018, the entire contents of which areincorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor memorydevice.

BACKGROUND

A semiconductor memory in which memory cells are three-dimensionallystacked is known.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system which utilizes a semiconductormemory device according to a first embodiment;

FIG. 2 is a circuit diagram of a memory cell array of a semiconductormemory device according to the first embodiment;

FIGS. 3 and 4 show layout views of selection gate lines and word linesof a semiconductor memory device according to the first embodiment;

FIG. 5 shows a vertical section of a block of a semiconductor memorydevice according to the first embodiment;

FIGS. 6 and 7 show cross and vertical sections of a memory celltransistor of a semiconductor memory device according to the firstembodiment;

FIG. 8 shows an equivalent circuit of transistors formed in a pillar ofa semiconductor memory device according to the first embodiment;

FIG. 9 schematically shows a relation of stored bit values and thresholdvoltage distribution for a memory cell of a semiconductor memory deviceaccording to the first embodiment;

FIG. 10 shows an order of writing data in a semiconductor memory deviceaccording to the first embodiment;

FIG. 11 shows a flowchart of a write operation of a semiconductor memorydevice according to the first embodiment;

FIG. 12 shows a timing chart of transition of voltages of signal linesduring a write operation in a semiconductor memory device according tothe first embodiment;

FIG. 13 shows pre-verify target string units and pre-program targetstring units in a write operation for each write target string unit in asemiconductor memory device according to the first embodiment;

FIG. 14 shows a flowchart of a data write operation in a semiconductormemory device according to a second embodiment;

FIG. 15 shows a timing chart of the transition of voltages of signallines during a write operation in a semiconductor memory deviceaccording to the second embodiment;

FIG. 16 shows a flowchart of a write operation in a semiconductor memorydevice according to a third embodiment;

FIG. 17 shows a timing chart of the transition of voltages of signallines during a write operation in a semiconductor memory deviceaccording to the third embodiment;

FIG. 18 shows a data writing order in a semiconductor memory deviceaccording to a fourth embodiment;

FIG. 19 shows a flowchart of a write operation in a semiconductor memorydevice according to the fourth embodiment;

FIG. 20 shows pre-verify target string units and pre-program targetstring units in a write operation for each write target string unit in asemiconductor memory unit according to the fourth embodiment;

FIG. 21 shows a command sequence for writing data into a semiconductormemory device according to the fourth embodiment;

FIG. 22 shows a data writing order in a semiconductor memory deviceaccording to a modification example of the fourth embodiment;

FIGS. 23 and 24 show the threshold voltage transition during a datawrite operation for memory cells in a semiconductor memory deviceaccording to a fifth embodiment;

FIG. 25 shows a data writing order in a semiconductor memory deviceaccording to the fifth embodiment;

FIG. 26 shows a flowchart of a write operation in a semiconductor memorydevice according to the fifth embodiment;

FIG. 27 shows pre-verify target string units and pre-program targetstring units in the write operation for each write target string unit ina semiconductor memory device according to the fifth embodiment;

FIGS. 28, 29, 30, 31, 32, 33, 34, and 35 show first to eighth examplesof a data writing order in a semiconductor memory device according to asixth embodiment, respectively;

FIG. 36 shows a first example of a data writing order in a semiconductormemory device according to a seventh embodiment;

FIG. 37 shows a second example of a data writing order in asemiconductor memory device according to the seventh embodiment; and

FIG. 38 shows a third example of a data writing order in a semiconductormemory device according to the seventh embodiment.

DETAILED DESCRIPTION

in general, according to one embodiment, a semiconductor memory deviceincludes: a first memory cell; a second memory cell; a first word line;a second word line; and a first bit line. The first memory cell iscapable of holding data. The second memory cell is capable of holdingdata. The first word line is coupled to the first memory cell. Thesecond word line is coupled to the second memory cell. The first bitline is capable of being electrically coupled to both the first memorycell and the second memory cell. The first memory cell shares asemiconductor layer with the second memory cell, and faces the secondmemory cell across the semiconductor layer. The semiconductor memorydevice is configured to execute a first operation, a second operation,and a third operation to write data into the first memory cell. In thefirst operation, a first voltage is applied to the second word line. Inthe second operation, after the first operation, a second voltage higherthan the first voltage is applied to the second word line. In the thirdoperation, after the second operation, a third voltage higher than thesecond voltage is applied to the first word line, and a fourth voltagelower than both the second voltage and the third voltage is applied tothe second word line.

1. First Embodiment

A semiconductor memory device according to the first embodiment will beexplained. In the explanation below, a memory system which uses a NANDflash memory as the semiconductor memory device will be discussed by wayof example.

1.1 Configuration

First, the configuration of the memory system according to the presentembodiment will be explained.

1.1.1 Overall Configuration

The overall configuration of the memory system according to the presentembodiment will be discussed with reference to FIG. 1.

As illustrated in FIG. 1, the memory system 1 includes a NAND flashmemory 100 and a controller 200. The NAND flash memory 100 and thecontroller 200 may be combined to constitute a single semiconductorequipment, including but not limited to a memory card such as an SDcard, and a solid state drive (SSD). The controller 200 may be asystem-on-chip (SoC) controller.

The NAND flash memory 100 includes a plurality of memory cells to storedata in a non-volatile manner. The controller 200 is connected to theNAND flash memory 100 via a NAND bus, and is also connected to a hostapparatus 300 via host bus. The controller 200 controls the NAND flashmemory 100, and accesses the NAND flash memory 100 in response to aninstruction received from the host apparatus 300. The host apparatus 300may be, for example, a digital camera or a personal computer, and thehost bus may be, for example, of the bus architecture in compliance withthe SD™ interface. The NAND bus is used to exchange signals in a mannerin compliance with the NAND interface.

1.1.2 Configuration of Controller 200

Continuing to refer to FIG. 1, the configuration of the controller 200will be explained in detail. The controller 200 of FIG. 1 includes ahost interface circuit 210, an internal memory (RAM) 220, a processor(CPU) 230, a buffer memory 240, a NAND interface circuit 250, and an ECC(Error Checking and Correcting) circuit 260.

The host interface circuit 210 is connected to the host apparatus 300via the host bus, and transfers instructions and data received from thehost apparatus 300 to the processor 230 and to the buffer memory 240,respectively. Furthermore, in response to an instruction from theprocessor 230, the host interface circuit 210 transfers data stored inthe buffer memory 240 to the host apparatus 300.

The processor 230 controls operations of the controller 200. Forinstance, upon receipt of a data write instruction from the hostapparatus 300, the processor 230 sends a write command to the NANDinterface circuit 250. The data reading and the data erasing areperformed in similar ways. The processor 230 implements variousoperations, such as wear leveling, to manage the NAND flash memory 100.The operation of the controller 200 discussed below may be implementedby one or more processors executing a software or firmware program, ormay be realized by hardware.

The NAND interface circuit 250 is connected to the NAND flash memory 100via the NAND bus, and controls communications with the NAND flash memory100. In response to an instruction received from the processor 230, theNAND interface circuit 250 transmits various signals to the NAND flashmemory 100, and receives various signals from the NAND flash memory 100.

The buffer memory 240 temporarily stores data to be written in the NANDflash memory 100 (write data) and data read out from the NAND flashmemory 100 (read data).

The internal memory 220 includes a semiconductor memory such as adynamic random access memory (DRAM) and a static random access memory(SRAM), and is used as a workspace for the processor 230. The internalmemory 220 stores firmware for controlling the NAND flash memory 100,and data tables for various control and/or management operations.

The ECC circuit 260 executes error detection and error correctionoperations for the data stored in the NAND flash memory 100. Whenwriting data into the NAND flash memory 100, the ECC circuit 260generates error correction codes and associates the error correctioncodes with the write data. When reading out data, the ECC circuit 260decodes the error correction codes of the read data.

1.1.3 Configuration of NAND Flash Memory 100 1.1.3.1 OverallConfiguration of NAND Flash Memory 100

Next, the configuration of the NAND flash memory 100 is explained. TheNAND flash memory 100 of FIG. 1 includes a memory cell array 110, a rowdecoder 120, a driver circuit 130, a sense amplifier 140, an addressregister 150, a command register 160, and a sequencer 170.

The memory cell array 110 includes a plurality of blocks BLK, whichinclude a plurality of non-volatile memory cells each associated with arow and a column. The memory cell array 110 stores the data suppliedfrom the controller 200.

The row decoder 120 selects one of the blocks BLK, and further selectsone or more rows of the selected block BLK.

The driver circuit 130 applies a voltage to the selected block BLK viathe row decoder 120.

The sense amplifier 140, at the time of reading data or verifying thedata, senses the data read from the memory cell array 110 and performsthe necessary computations based on the sensed data. This read data DATis output to the controller 200. At the time of writing data, the senseamplifier 140 transfers the write data DAT received from the controller200, to the memory cell array 110.

The address register 150 holds an address ADD received from thecontroller 200. The command register 160 holds a command CMD receivedfrom the controller 200.

The sequencer 170 controls operations of the NAND flash memory 100,based on various types of information held in the registers 150 and 160.

1.1.3.2 Configuration of Memory Cell Array 110

The memory cell array 110 of the semiconductor memory device accordingto the present embodiment will be explained.

<Circuit Configuration>

First, the circuit configuration of the memory cell array 110 will beexplained with reference to FIG. 2. FIG. 2 shows an equivalent circuitof one of the blocks ELK. As illustrated in FIG. 2, the block ELKincludes a plurality of string units SU (SU0, SU1, SU2, . . . ), andeach string unit SU includes a plurality of NAND strings 50.

Hereinafter, to differentiate the NAND strings of the string units witheven numbers (even string units) SUe (SU0, SU2, SU4, . . . ) from theNAND strings of the string units with odd numbers (odd string units) SUo(SU1, SU3, SU5, . . . ), they will be collectively referred to as NANDstrings 50 e and NAND strings 50 o, respectively.

Each of the NAND strings 50 may include eight memory cell transistors MT(MT0 to MT7) and selection transistors ST1 and ST2. Each memory celltransistor MT includes a control gate and a charge accumulation layer,and holds data in a non-volatile manner. The eight memory celltransistors MT are coupled in series between the source of the selectiontransistor ST1 and the drain of the selection transistor ST2.

The gates of the selection transistors ST1 in each string unit SU arecoupled to a single selection gate line SGD (SGD0, SGD1, . . . ). Eachof the selection gate lines SGD is independently controlled by the rowdecoder 120. The gates of the selection transistors ST2 in each of theeven string units SUe (SU0, SU2, . . . ) may be commonly coupled to theselection gate line SGSe, and the gates of the selection transistors ST2in each of the odd string units SUo (SU1, SU3, . . . ) may be commonlycoupled to the selection gate line SGSo. The selection gate lines SGSeand SGSo may be controlled either commonly or independently.

The control gates of the memory cell transistors MT (MT0 to MT7)included in the even string units SUe of the same block BLK are commonlycoupled to the corresponding word lines WLe (WLe0 to WLe7),respectively. The control gates of the memory cell transistors MT (MT0to MT7) included in the odd string unit SUo are commonly coupled to thecorresponding word lines WLo (WLo0 to WLo7), respectively. The wordlines WLe and the word lines WLo are independently controlled by the rowdecoder 120.

A block BLK may be a minimum size unit for data erasing. In other words,the data stored in all the memory cell transistors MT in the same blockBLK is erased at a time. However, the data may be erased in every stringunit SU, or in units smaller than a string unit SU.

In the memory cell array 110, the drains of the selection transistorsST1 of the NAND strings 50 in the same column are commonly coupled tothe same bit line BL (BL0 to BL(L−1), where L is a natural number largerthan or equal to 2). That is, a bit line BL is commonly coupled to theNAND strings 50 e and 50 o in the same column across different stringunits SU. The sources of all of the selection transistors ST2 arecommonly coupled to a source line SL.

In other words, each string unit SU includes a plurality of NAND strings50 that are coupled to their respective bit lines BL and to the sameselection gate line SGD. Each block BLK includes a plurality of stringunits SU that share the word lines WL. A memory cell array 110 includesa plurality of blocks BLK that share bit lines BL. In the memory cellarray 110, the memory cell transistors MT and the selection transistorsST1 and ST2 are three-dimensionally stacked by forming

the selection gate line SGS, the word lines WL, and the selection gateline SGD in layers on the semiconductor substrate.

<Planar Layout of Memory Cell Array>

Next, the planar layout of the memory cell array 110 is described. FIG.3 shows a planar layout of the selection gate lines SGD of a certainblock BLK along the plane of the semiconductor substrate (referred to asan “X-Y plane”). In this example, one block BLK includes eight selectiongate lines SGD (SDG0 to SGD7).

As illustrated in FIG. 3, 17 conductive layers (interconnect lines) 10(10-0 a to 10-7 a, 10-0 b to 10-7 b, and 10-0 c) extending along the Xdirection are arranged in the Y direction orthogonal to the X direction.Each conductive layer 10 functions as the selection gate line SGD. Inthe example of FIG. 3, the conductive layers 10-0 a, 10-1 a, 10-2 a,10-1 b, 10-2 b, 10-3 a, 10-4 a, 10-3 b, 10-4 b, 10-5 a, 10-6 a, 10-5 b,10-6 b, 10-7 a, 10-0 b, 10-7 b, and 10-0 c are arranged in the Ydirection. The two conductive layers 10-0 a and 10-0 c positioned at theopposite ends of the block BLK in the Y direction, and the conductivelayer 10-0 b positioned between the conductive layers 10-7 a and 10-7 bare electrically coupled to each other to function as the selection gateline SGD0. Furthermore, the conductive layers 10-1 a and 10-1 b areelectrically coupled to each other to function as the selection gateline SGD1. The conductive layers 10-2 a and 10-2 b are electricallycoupled to each other to function as the selection gate line SGD2, andthe conductive layers 10-3 a and 10-3 b are electrically coupled to eachother to function as the selection gate line SGD3. The same arrangementis applied to other conductive layers for the other selection gate linesSGD4 to SGD7. The conductive layers 10-4 a and 10-4 b are electricallycoupled to each other to function as the selection gate line SGD4; theconductive layers 10-5 a and 10-5 b are electrically coupled to eachother to function as the selection gate line SGD5; the conductive layers10-6 a and 10-6 b are electrically coupled to each other to function asthe selection gate line SGD6; and the conductive layers 10-7 a and 10-7b are electrically coupled to each other to function as the selectiongate line SGD7.

In the block BLK, the conductive layers 10 that are adjacent to eachother in the Y direction are separated by an insulating layer (notshown). The regions in which the insulating layers are located arereferred to as slits SLT2. In each slit SLT2, the insulating layer isburied from the surface of the semiconductor substrate to at least thelayer where the conductive layers 10 are arranged. Furthermore, in thememory cell array 110, a plurality of blocks BLK as illustrated in FIG.3 are arranged in the Y direction. The blocks BLK that are adjacent toeach other in the Y direction are also separated by an insulating layer(not shown). The regions in which these insulating layers are locatedare referred to as slits SLT1.

A plurality of pillars MP (MP0 to MP31) extending along the Z directionare arranged between the conductive layers 10 that are adjacent to eachother in the Y direction to form the selection transistors ST1 and ST2and the memory cell transistors MT. The plurality of pillars arereferred to as memory pillars. The Z direction is orthogonal to the X-Yplane, or in other words, vertical to the surface of the semiconductorsubstrate.

In particular, the memory pillars MP0 and MP16 are arranged between theconductive layers 10-0 a and 10-1 a; the memory pillars MP8 and MP24 arearranged between the conductive layers 10-1 a and 10-2 a; the memorypillars MP1 and MP17 are arranged between the conductive layers 10-2 aand 10-1 b; and the memory pillars MP9 and MP25 are arranged between theconductive layers 10-1 b and 10-2 b. Furthermore, the memory pillars MP2and MP18 are arranged between the conductive layers 10-2 b and 10-3 a;the memory pillars MP10 and MP26 are arranged between the conductivelayers 10-3 a and 10-4 a; the memory pillars MP3 and MP19 are arrangedbetween the conductive layers 10-4 a and 10-3 b; and the memory pillarsMP11 and MP27 are arranged between the conductive layers 10-3 b and 10-4b. The memory pillars MP4 and MP20 are arranged between the conductivelayers 10-4 b and 10-5 a; the memory pillars MP12 and MP28 are arrangedbetween the conductive layers 10-5 a and 10-6 a; the memory pillars MP5and MP21 are arranged between the conductive layers 10-6 a and 10-5 b;and the memory pillars MP13 and MP29 are arranged between the conductivelayers 10-5 b and 10-6 b. Also, memory pillars MP6 and MP22 are arrangedbetween the conductive layers 10-6 b and 10-7 a; the memory pillars MP14and MP30 are arranged between the conductive layers 10-7 a and 10-0 b;the memory pillars MP7 and MP23 are arranged between the conductivelayers 10-0 b and 10-7 b; and the memory pillars MP15 and MP31 arearranged between the conductive layers 10-7 b and 10-0 c. The memorypillars MP are structures that are used to form the selectiontransistors ST1 and ST2 and the memory cell transistors MT, which willbe discussed in detail later.

The memory pillars MP0 to MP7 are located along the Y direction. Thememory pillars MP16 to MP23 are located along the Y direction side byside with the memory pillars MP0 to MP7 in the X direction. That is, thealignment of the memory pillars MP0 to MP7 is parallel to the alignmentof the memory pillars MP16 to MP23.

Similarly, the memory pillars MP8 to MP15 are located along the Ydirection, and the memory pillars MP24 to MP31 are also located alongthe Y direction. The alignment of the memory pillars MP8 to MP15 ispositioned between the alignment of the memory pillars MP0 to MP7 andthe alignment of the memory pillars MP16 to MP23 with respect to the Xdirection. The alignment of the memory pillars MP24 to MP31 ispositioned in such a manner that the alignment of the memory pillarsMP16 to MP23 is interposed between the alignment of the memory pillarsMP24 to MP31 and the alignment of the memory pillars MP8 to MP15, withrespect to the X direction. The alignment of the memory pillars MP8 toMP15 is positioned parallel to the alignment of the memory pillars MP24to MP31.

The memory pillars MP0 to MP7 and the memory pillars MP16 to MP23 arearranged to form a staggered pattern together with the memory pillarsMP8 to MP15 and the memory pillars MP24 to MP31. In particular, theformer memory pillars MP0 to MP7 and MP16 to MP23 are shifted from thelatter memory pillars MP8 to MP15 and MP24 to MP31 by one slit intervalof the slits SLT2 in the Y direction. In other words, each memory pillarMP is buried between two adjacent conductive layers 10 in the Ydirection and in part of the corresponding slit SLT2. Thus, one slitSLT2 is present between any two memory pillars MP that are adjacent toeach other in the Y direction. For two adjacent blocks BLK, no memorypillar MP is provided between the conductive layer 10-0 a of one blockand the conductive layer 10-0 c of the other block that face each otheracross the slit SLT1.

Two bit lines BL are arranged above a certain memory pillar MP. Of thetwo bit lines BL, only one bit line BL is electrically coupled to thatmemory pillar MP.

For example, two bit lines BL0 and BL1 are arranged above the memorypillars MP0 to MP7. The bit line BL0 is commonly coupled to the memorypillars MP0, MP2, MP4, and MP6, while the bit line BL1 is commonlycoupled to the memory pillars MP1, MP3, MP5, and MP7. Two bit lines BL2and BL3 are arranged above the memory pillars MP8 to MP15. The bit lineBL2 is commonly coupled to the memory pillars MP8, MP10, MP12, and MP14,while the bit line BL3 is commonly coupled to the memory pillars MP9,MP11, MP13, and MP15. Two bit lines BL4 and BL5 are arranged above thememory pillars MP16 to MP23. The bit line BL4 is commonly coupled to thememory pillars MP16, MP18, MP20, and MP22, while the bit line BL5 iscommonly coupled to the memory pillars MP17, MP19, MP21, and MP23. Twobit lines BL6 and BL7 are arranged above the memory pillars MP24 toMP31. The bit line BL6 is commonly coupled to the memory pillars MP24,MP26, MP28, and MP30, while the bit line BL7 is commonly coupled to thememory pillars MP25, MP27, MP29, and MP31.

Like FIG. 3, FIG. 4 shows a planar layout of the word lines WL on theX-Y plane. The layout of FIG. 4 corresponds to the one block illustratedin FIG. 3, showing conductive layers 11 which is arranged below theconductive layers 10 shown in FIG. 3.

As illustrated in FIG. 4, 17 conductive layers 11 (11-0 to 11-16)extending along the X direction are arranged in the Y direction. Theconductive layers 11-0 to 11-16 are positioned immediately below theconductive layers 10-0 a, 10-1 a, 10-2 a, 10-1 b, 10-2 b, 10-3 a, 10-4a, 10-3 b, 10-4 b, 10-5 a, 10-6 a, 10-5 b, 10-6 b, 10-7 a, 10-0 b, 10-7b, and 10-0 c, with an insulating layer interposed therebetween. Each ofthe conductive layers 11 functions as the word line WLe7 or WLo7. Theother word lines WL0 to WL6 are configured in a similar manner.

In the example of FIG. 4, the conductive layers 11-0, 11-2, 11-4, 11-6,11-8, 11-10, 11-12, 11-14, and 11-16 extend to their respective ends ina first direction along the X direction (referred to as first connectionends). The conductive layers 11-0, 11-14, and 11-16 are coupledtogether, the conductive layers 11-2 and 11-4 are coupled together, theconductive layers 11-6 and 11-8 are coupled together, and the conductivelayers 11-10 and 11-12 are coupled together. Each of the conductivelayers 11-0, 11-2, 11-4, 11-6, 11-8, 11-10, 11-12, 11-14, and 11-16functions as the word line WLe7.

The conductive layers 11-1, 11-3, 11-5, 11-7, 11-9, 11-11, 11-13, and11-15 extend to their respective ends (referred to as second connectionends) in a second direction opposite to the first direction along the Xdirection. The conductive layers 11-1 and 11-3 are coupled together, theconductive layers 11-5 and 11-7 are coupled together, the conductivelayers 11-9 and 11-11 are coupled together, and the conductive layers11-13 and 11-15 are coupled together, each of which functions as theword line WLo7.

The memory cell transistors MT are provided between the first connectionends and the second connection ends. In regions in which the memory celltransistors MT are formed (memory cell regions), the conductive layers11 which are adjacent to each other in the Y direction are separated bythe slits SLT2 described by referring to FIG. 3. Similarly, between theblocks BLK which are adjacent to each other in the Y direction, theconductive layer 11 of one block BLK and the conductive layer of theother block BLK which are adjacent to each other in the Y direction areseparated by the slits SLT1. Furthermore, the memory pillars MP0 to MP31are located in the memory cell regions as explained with reference toFIG. 3.

The layers in which the other word lines WL and the selection gate linesSGS are configured similarly to the configuration of the above-describedlayer.

<Cross-sectional Structure of Memory Cell Array>

The cross-sectional structure of the memory cell array 110 is nowdiscussed. FIG. 5 shows a vertical section of the block BLK, taken alongthe Y direction. By way of example, FIG. 5 illustrates a verticalsection of a region taken along the bit line BL0 shown in FIG. 3.

As illustrated in FIG. 5, the conductive layers 12 functioning as theselection gate lines SGS is located above the semiconductor substrate(e.g., p-type well region) 13. Eight conductive layers 11 functioning asthe word lines WL0 to WL7 are stacked in the Z direction above theselection gate lines SGS. The layout views of these conductive layersfor the word lines 11 and the SGS 12 are presented in FIG. 4. Theconductive layers 10 functioning as the selection gate lines SGD isarranged above the word lines 11. As for the layer for the SGD, itslayout view has already been discussed with reference to FIG. 3.

The slits SLT2 and the memory pillars MP are arranged alternately alongthe Y direction to each extend through the SGD 10 to the semiconductorsubstrate 13. As discussed above, the slits SLT2 are formed ofinsulating layers. The insulation films of the slit SLT2 may includecontact plugs to apply a voltage to a region of the semiconductorsubstrate 13. For example, contact plugs may be provided to couple thesources of the selection transistors ST2 to a source line that is notshown.

The layer for the SGS 12 includes the selection gate lines SGSe and theselection gate lines SGSo which are alternately arranged with slits SLT2or memory pillars MP interposed in-between. Similarly, each of thelayers for the word lines WL 11 includes the word lines WLo and the wordlines WLe which are alternately arranged with slits SLT2 or memorypillars MP interposed therebetween.

The slits SLT1 are arranged between the blocks BLK which are adjacent toeach other in the Y direction. As mentioned above, the slits SLT1 areformed of an insulating layer. The insulating layers of the slits SLT1may include contact plugs to apply a voltage to a region of thesemiconductor substrate 13. For example, contact plugs may be providedto couple the sources of the selection transistors ST2 to the sourceline. The width of the slit SLT1 in the Y direction is greater than thewidth of the slit SLT2 in the Y direction.

A contact plug 16 is arranged on each of the memory pillars MP. Theconductive layers 15 functioning as the bit lines BL is arranged alongthe Y direction in a manner so that the bit lines BL are commonlycoupled to the corresponding contact plugs 16.

<Structure of Memory Pillar and Memory Cell Transistor>

The structure of each memory pillar MP and each memory cell transistorMT will be explained. FIG. 6 is a cross section of the memory pillar MPtaken along the X-Y plane, and FIG. 7 is a vertical section of the sametaken along the Y-Z plane. Particularly, FIGS. 6 and 7 illustrate thearea in which two memory cell transistors MT are formed.

As illustrated in FIGS. 6 and 7, the memory pillar MP includes aninsulating layer 30, a semiconductor layer 31, and insulating layers 32to 34 which are coaxially arranged along the Z direction. The insulatinglayer 30 may be a silicon dioxide film. The semiconductor layer 31 isarranged to surround the insulating layer 30, and provides a region inwhich the channels of the memory cell transistors MT are formed. Thesemiconductor layer 31 may be formed of polycrystalline silicon. Theinsulating layer 32 is arranged to surround the semiconductor layer 31,and to function as a gate insulating layer of the memory cell transistorMT. The insulating layer 32 may have a stacked structure including asilicon dioxide film and a silicon nitride film. The insulating layer 33is arranged to surround the semiconductor layer 31, and functions as acharge accumulation layer of the memory cell transistor MT. Theinsulating layer 33 may be a silicon nitride film. The insulating layer34 is arranged to surround the insulating layer 33, and functions as ablock insulating layer of the memory cell transistor MT. The insulatinglayer 34 may be a silicon dioxide film. Outside the memory pillar MP, aninsulating layer 37 is buried in the slit SLT2. The insulating layer 37may be a silicon dioxide film.

An aluminum oxide (AlO) layer 35 may be arranged around theabove-described memory pillar MP. A barrier metal layer (e.g., atitanium nitride (TiN) film) 36 may be formed around the AlO layer 35.The conductive layers 11 that function as the word lines WL are arrangedaround the barrier metal layer 36. The conductive layers 11 may beformed of tungsten.

With the above-described configuration, two memory cell transistors MTare formed along the Y direction in one region of the memory pillar MPshown in FIGS. 6 and 7. The selection transistors ST1 and ST2 aresimilarly formed.

FIG. 8 shows an equivalent circuit of the transistors formed in thememory pillar MP. As illustrated in FIG. 8, two NAND strings 50 o and 50e are formed in one memory pillar MP. In other words, a pair ofselection transistors ST1 arranged in the one memory pillar MP arecoupled to different selection gate lines SGD. The pair of memory celltransistors MT0 to MT7 are respectively coupled to different word linesWLo and WLe. Furthermore, the pair of selection transistors ST2 arecoupled to different selection gate lines SGSo and SGSe. The two NANDstrings 50 o and 50 e in the same memory pillar MP are coupled to thesame bit line BL, and to the same source line SL. The two NAND strings50 o and 50 e formed in the same memory pillar MP share a back gate(semiconductor layer 31).

<Threshold Voltage Distribution of Memory Cell Transistor>

In an example introduced here, a single memory cell transistor MT may beconfigured to hold 3-bit data. The bits of this 3-bit data will bereferred to, from the least significant bit, as a lower bit, middle bit,and upper bit. In the memory cell transistors that are coupled to thesame word line, a set of lower bits held in those memory celltransistors is denoted as a lower page, a set of middle bits held in thethose memory cells transistors is denoted as a middle page, and a set ofupper bits held in the those memory cells transistors is denoted as anupper page. That is, three pages are assigned to one word line WL, whichmeans that one string unit SU coupled to eight word lines WL has acapacity of 24 pages. In other words, a “page” can be defined as aportion of the memory space that is formed by the memory celltransistors coupled to the same word line. The data writing and readingmay be performed for every page.

FIG. 9 shows the possible bit values to be stored in each memory celltransistor MT, threshold voltage distribution, and read voltages usedfor reading data.

As shown in FIG. 9, one memory cell transistor MT may be set in one ofeight possible states in accordance with the threshold voltage states.These eight states are referred to as “Er” state, “A” state, “B” state,“C” state, . . . and “G” state from the lowest threshold voltage state.

The threshold voltage of the memory cell transistor MT in the “Er” stateis larger than the voltage Vcut1 (which may be a negative voltage) andlower than the read voltage VA, which corresponds to the data erasedstate. The read voltage VA may be 0V, and the threshold voltage of thememory cell transistor MT in the “Er” state is negative. The thresholdvoltage of the memory cell transistor MT in the “A” state is greaterthan or equal to the read voltage VA, and is smaller than the readvoltage VB (>VA). The threshold voltage of the memory cell transistor MTin the “B” state is greater than or equal to the read voltage VB, and issmaller than the read voltage VC (>VB). The threshold voltage of thememory cell transistor MT in the “C” state is greater than or equal tothe read voltage VC, and is smaller than the read voltage VD (>VC). Thethreshold voltage of the memory cell transistor MT in the “D” state isgreater than or equal to the read voltage VD, and is smaller than theread voltage VE (>VD). The threshold voltage of the memory celltransistor MT in the “E” state is greater than or equal to the readvoltage VE, and is smaller than the read voltage VF (>VE). The thresholdvoltage of the memory cell transistor MT in the “F” state is greaterthan or equal to the read voltage VF, and is smaller than the readvoltage VG (>VF). The threshold voltage of the memory cell transistor MTin the “G” state is greater than or equal to the read voltage VG, and issmaller than the voltage VREAD. Of these eight states corresponding tothe above-described threshold voltage distribution states, the “G” stateindicates the highest threshold voltage distribution. The voltage VREADis a voltage which is applied to non-selected word lines at the time ofdata reading and turns on any memory cell transistors MT coupled to thenon-selected word lines regardless of the data stored in those memorycell transistors.

The above-described threshold voltage distribution states are realizedby writing a 3-bit (3-page) value including the lower bit, middle bit,and upper bit in the corresponding memory cell transistor. The eightthreshold voltage distribution states establish the followingrelationship with the lower bit, middle bit, and upper bit values.

“Er” state: “111” (in the order of “upper/middle/lower”)“A” state: “110”“B” state: “100”“C” state: “000”“D” state: “010”“E” state: “011”“F” state: “001”“G” state: “101”

In the above threshold voltage distribution states, the bit values ofany adjacent two states have a difference in only one of the three bits.

In view of the above, when reading the value of the lower bit, the readvoltages corresponding to the boundary voltages at which the value (“0”or “1”) of the lower bit changes are used. The same applies to themiddle bit and the upper bit.

1.2 Write Operation

A method of writing data into the NAND flash memory having theabove-described configuration will be explained. In the method ofprogramming data in the semiconductor memory device according to thepresent embodiment, the data of three pages as explained with referenceto FIG. 9 is received, and the programming is executed based on thisdata, thereby directly changing the threshold voltage of the memory celltransistor MT from the “Er” state to a targeted threshold voltage.Hereinafter, such a method is referred to as a full sequenceprogramming.

1.2.1 Writing Order

First, the selecting order of the string units SU0 to SU7 and word linesWL0 to WL7 when writing data into a certain block BLK is explained withreference to FIG. 10. FIG. 10 shows the selecting order, where thenumerals “0” to “63” denote the places in the selecting order, which areindicated in cells identified by the numbers of the string units SU0 toSU7 in the top row and the numbers of the word lines WL0 to WL7 in theleft column.

In the example of FIG. 10, a higher priority is assigned to theselection of a word line WL than the selection of a string unit SU. Thatis, the word line WL0 of the string unit SU0 is first selected, then theword line WL0 of the string unit SU is selected, and thereafter the wordlines WL0 of the string unit SU2 to SU7 are selected in the same manner.After the word line WL0 of the last string unit SU7 is selected, theselection proceeds to the word line WL1. Then, the word lines WL1 of thestring units SU0 to SU7 are sequentially selected. In this manner, Forthe word lines WL2 to WL7 in turn, the selection are performed in thesame manner, and finally the word line WL7 of the string unit SU7 isselected.

1.2.2 Details of Write Operation

Next, the write operation will be described in detail with reference toFIG. 11. FIG. 11 illustrates a flowchart of the write operation.

First, the NAND flash memory 100 receives a write command from thecontroller 200 (step S10). The write command is stored in the commandregister 160. An address sent together with the write command from thecontroller 200 is stored in the address register 150, and the write datais transferred to the sense amplifier 140. As described above, data ofthree pages is held in the sense amplifier 140.

With the write command stored in the command register 160, the sequencer170 initiates the write operation. Roughly describing, the writeoperation includes three operations, namely, a pre-verify operation, apre-program operation, and a data program operation. The operations areexplained below with reference to the timing chart of FIG. 12, showingtransition of voltages of the signal lines. In the example of FIG. 12,the word line WLi (where i is a variable between 0 and 7) of the stringunit SU0 is designated by the controller 200.

First, the sequencer 170 performs the pre-verify operation.

<Pre-verify Operation>

First, the sequencer 170 selects the string unit SU designated by theaddress that is received from the controller 200 (step S11). Then, thesequencer 170 reads data from the string unit SU selected at step S11.Here, the word line WL designated by the controller 200 is selected, andthe voltage Vcut1 is applied to the selected word line WL as a readvoltage (step S12).

Next, the sequencer 170 selects one or more other string units SU thatshares any memory pillar MP with the string unit SU designated by theaddress that is received from the controller 200 (step S13). Then, thesequencer 170 reads the data from the one or more other string units SUselected at step S13. Again, the word line WL designated by thecontroller 200 is selected, and the voltage Vcut1 is applied to theselected word line WL as a read voltage (step S14). In a case where aplurality of string units SU share the memory pillars MP with the stringunit SU designated by the address, steps S13 and S14 are performed foreach of the plurality of string units SU.

The timing chart during the above-described steps S11 and S12 are shownbetween times t0 and t1 in FIG. 12. In this example, the controller 200designates the word line WLi of the string unit SU0. The row decoder 120applies a voltage VSG to the selection gate line SGD0 (and also to theselection gate line SGSe) corresponding to the string unit SU0. Thevoltage VSG turns on the selection transistors ST1 and ST2. The rowdecoder 120 further selects the word line WLi (WLei) and applies thevoltage Vcut1 to the word line WLi. In addition, the row decoder 120applies the voltage Vcut2 to the non-selected word lines WL(i±1) whichare adjacent to the word line WLi, and applies the voltage VREAD toother non-selected word lines WL. The voltage Vcut2 may be a negativevoltage, where the absolute value of Vcut2 is equal to and less than theabsolute value of Vcut1, |Vcut1|≥|Vcut2|. The voltage VREAD has anamount of voltage enough to turn a memory cell transistor MT on,regardless of the data held in the memory cell transistor. The senseamplifier 140 pre-charges bit lines BL (BL0 to BL(L−1)). As a result, ifa memory cell transistor MTi of the string unit SU0 is turned on, thememory cell transistor MTi passes a cell current from the correspondingbit line BL to the source line SL. The cell current can be detected bythe sense amplifier 140. If a cell current flows through any of the bitlines BL, it indicates that the threshold value of the memory celltransistor MT coupled to the bit line and the word line WLi in thestring unit SU0 is smaller than or equal to Vcut1. Hereinafter, such amemory cell transistor MT may be referred to as an “over-erased celltransistor”.

Next, the states of the signal lines during steps S13 and S14 are shownbetween times t1 and t2 in FIG. 12. As illustrated in FIG. 12, theoperations executed between times t0 and t1 are performed also for thestring units SU1 and SU7. For instance, the row decoder 120 firstapplies the voltage VSG to the selection gate line SGD1 (and also to theselection gate line SGSo) corresponding to the string unit SU1, appliesthe voltage Vcut1 to the selected word line WLi (WLoi in this example),applies the voltage Vcut2 to the non-selected word lines WL(i±1) whichare adjacent to the selected word line WLi, and applies the voltageVREAD to other non-selected word lines WL. The string unit SU1 sharesthe memory pillars MP0 and MP16 with the string unit SU0 (see FIG. 4).The memory pillars MP0 and MP16 are coupled to the bit lines BL0 and BL4(see FIG. 3). That is, the string unit SU0 shares the memory pillars MPcoupled to the bit line BL(4 j) with the string unit SU1, where 4jdenotes a multiplication of 4 and j, and j is an integer larger than orequal to zero. Thus, at the time of pre-verifying for the string unitSU, the sense amplifier 140 pre-charges the bit lines BL(4 j) coupled tothose shared memory pillars MP, and senses the currents flowing throughthe bit lines BL(4 j) or the voltages thereof, while the other bit linesBL(4 j+1), BL(4 j+2), and BL(4 j+3) (i.e. BL1, BL2, BL3, BL5, BL6, . . .) may be fixed to the voltage VSS. If the cell current flows from anyone of the bit lines BL(4 j) to the source line SL, it indicates thatthe threshold voltage value of at least one of the memory celltransistors MT coupled to the word line WLi (eg. conductive layer 11-1)and the one bit line BL(4 j) in the string unit SU1 is smaller than orequal to Vcut1.

Similarly, the pre-verify operation is executed for the string unit SU7.The string unit SU7 has an only difference from the string unit SU1 inthat the string unit SU7 shares the memory pillars MP (in the example ofFIGS. 3 and 4, memory pillars MP7, MP14, MP15, MP23, MP30 and MP31)coupled to the bit lines BL(4 j+1), BL(4 j+2), and BL(4 j+3) (in theexample of FIGS. 3 and 4, bit lines BL, BL2, BL3, BL5, BL6 and BL7),instead of the memory pillars MP(4 j) coupled to the bit line BL(4 j).These bit lines BL(4 j+1), BL(4 j+2) and BL(4 j+3) are pre-charged. Therest of the operations are the same as the operations for the stringunit SU1.

Next, the sequencer 170 performs the pre-program operation.

<Pre-Program Operation>

First, the sequencer 170 selects, from the string units SU selected atsteps S11 and S13, one or more string units including at least oneover-erased cell transistor (step S15). Then, the sequencer 170 executesthe program operation for the one or more string units SU selected atstep S15. In this operation, the word line WL designated by thecontroller 200 is selected, and the voltage VPGM_L is applied to theselected word line WL as a program voltage (step S16).

The states of the wiring lines during Steps S15 and S16 are shownbetween times t2 and t3 in FIG. 12. In this example, the string unitsSU0, SU1, and SU7 include over-erased cell transistors. The row decoder120 applies the voltage VSG to the selection gate lines SGD0, SGD1, andSGD7 corresponding to the string units SU0, SU1, and SU7, and thereafterapplies the voltage VSGD. The voltage VSGD has a lower voltage valuethan the voltage VSG. The voltage VSGD may turn the selection transistorST1 on when 0V is applied to the bit line BL, and turn the selectiontransistor ST1 off when the voltage VSG is applied to the bit line BL.The row decoder 120 further applies the voltage VSS to the selectiongate line SGS to turn the selection transistor ST2 off. Furthermore, therow decoder 120 selects the word lines WLi (WLei for the string unit SU0and WLoi for the string units SU1 and SU7 in this example), applies thevoltage VDD to these word lines WLi, and thereafter applies the voltageVPGM_L while applying the voltage VPASS to other non-selected word linesWL. The voltage VPGM_L has a lower voltage value than the programvoltage VPGM used for the data programming, where the relationshipVPGM>VPGM_L>VPASS>VSG>VDD is established. The sense amplifier 140applies 0V to the one or more bit lines BL through which the cellcurrent flows (i.e., the one or more bit lines to which at least oneover-erased cell transistor is coupled) in the pre-verify operation atsteps S11 to S15. On the other hand, a voltage VSG is applied to the oneor more bit lines BL through which the cell current does not flow in thepre-verify operation. In the string units SU0, SU1 and SU7, since chargeis injected into the charge accumulation layers of the memory celltransistors MTi of the NAND strings 50 corresponding to the one or morebit lines BL to which 0V is applied, the threshold voltage values ofthose memory cell transistors MTi are increased. Because the VPGM_L of alower voltage value than the VPGM which is used for ordinary dataprogramming is used, the shifts of the threshold values of those memorycell transistors MTi are smaller than in the ordinary data programming.The programming in the pre-programming operation is referred to as weakprogramming. In the NAND strings 50 corresponding to the one or more bitlines BL to which the voltage VSG is applied, the selection transistorsST1 are turned off. Thus, the weak programming is not applied to thememory cell transistors MTi of the string units SU0, SU1, and SU7corresponding to these bit lines BL.

Next, the sequencer 170 performs the data program operation.

<Data Program Operation>

First, the sequencer 170 selects the string unit SU designated by theaddress that is received from the controller 200 (step S17). The otherstring units SU remain non-selected. Then, the sequencer 170 executesthe program operation on the string unit SU selected at step S17. Forthis operation, the word line WL designated by the controller 200 isselected, and the voltage VPGM is used for a program voltage (step S18).The sequencer 170 performs the program verify operation (step S19). Ifthe program verify operation fails (NO at step S20), the sequencer 170increases a voltage value of the program voltage VPGM, and the processreturns to step S18.

The states of the wiring lines during Steps S18 and S19 are shownbetween times t3 and t5 in FIG. 12. The program operation is executedbetween times t3 to t4, and the program verify operation is executedbetween times t4 to t5.

As shown in FIG. 12, the row decoder 120 applies the voltage VSG to theselection gate line SGD0 corresponding to the string unit SU0, andthereafter applies the voltage VSGD to the selection gate line SGD0. Therow decoder 120 also applies the voltage VSS to the selection gate lineSGS to turn the selection transistor ST2 of the string unit SU0 off.Furthermore, the row decoder 120 selects a designated word line WLi(WLei for the string unit SU0 in this example), applies the voltage VDD,and thereafter the voltage VPGM (=VPGM_L+ΔV1), to this word line WLi,while applying the voltage VPASS(=VPGM_L-ΔV2) to other non-selected wordlines WL. The sense amplifier 140 applies 0V to the bit line BL0. In thestring unit SU0, since charge is injected into the charge accumulationlayer of the memory cell transistor MTi coupled to the selected wordline WLi and the bit line BL0, the threshold voltage value of the memorycell transistor MTi is increased.

Furthermore, the row decoder 120 applies the voltage VSS to theselection gate lines SGD1 to SGD7 of the non-selected string units SU1to SU7, thereby turning the selection transistors ST1 of thenon-selected string units SU1 to SU7 off. The row decoder 120 furtherapplies the voltage VPASS to the word lines WLi (i.e., WLoi) of thestring units SU and SU7, and the voltage VPASS to other non-selectedword lines WL.

Next, the program verify operation is executed between times t4 and t.As shown between the times t4 and t5 in FIG. 12, the row decoder 120applies the voltage VSG to the selection gate lines SGD0 and SGSe of thestring unit SU0 to turn its selection transistors ST1 and ST2 on. Therow decoder 120 selects the designated word line WLi (WLei for thestring unit SU0 in this example), applies the program verify voltageVvfy to this word line WLi, applies the voltage VREADK to thenon-selected word lines WL(i±1) that are adjacent to the selected wordline WLi, and applies the voltage VREAD to the other non-selected wordlines WL, where the relationship Vvfy<VREAD≤VREADK may be established.The row decoder 120 further applies the voltage VSS to the selectiongate lines SGD1 to SGD7 of the non-selected string unit SU to SU7 toturn their selection transistors ST1 off. The row decoder 120 appliesthe voltage Vcut1 to the word line WLi (i.e., WLoi) of the string unitsS1 and SU7, applies the voltage Vcut2 to the non-selected word linesWL(i±1) (i.e., WLo(i±1) for the string units SU1 and SU7), and appliesthe voltage VREAD to the other non-selected word lines WLo. The senseamplifier 140 pre-charges the bit lines BL. If cell currents do not flowfrom the bit lines BL to the source line SL, the program operationpasses the program verification, that is the data write operation wassuccessful.

The data write operation is performed in the above-described manner. Inthe example of FIG. 12, with the string unit SU0 selected, two stringunits SU1 and SU7 which share the memory pillars MP with the string unitSU0 are targeted for the pre-verify operation and the pre-programoperation. String units SU to be targeted for the pre-verify andpre-program operations, however, depend on which string unit SU isselected, as shown in FIG. 13. Further, the data writing or programmingorder as shown in FIG. 10 is also considered to select the target stringunits for the pre-verify and pre-program operations. It is because analready programmed string unit is not necessarily pre-verified forover-erased cell transistors even if the string unit shares any memorypillar with a write target string unit. The table of FIG. 13 showsstring units SU to be targeted for the pre-verify and pre-programoperations when writing data into each of the string units SU0 to SU7.In this table, “*1” indicates that the bit lines BL(4 j) arepre-verified, and “*2” indicates that the bit lines BL(4 j+1), BL(4j+2), and BL(4 j+3) are pre-verified. No mark indicates that the bitlines BL(4 j), BL(4 j+1), BL(4 j+2), and BL(4 j+3) are pre-verified.

As explained in the example of FIG. 12, when writing data into thestring unit SU0, the string units SU0, SU1, and SU7 are targeted for thepre-verify operation. Here, the bit lines BL to be targeted for thepre-verify operation are: all the bit lines BL for the string unit SU0;the bit line BL(4 j) for the string unit SU1; and the bit lines BL(4j+1), BL(4 j+2), and BL(4 j+3) for the string unit SU7.

Similarly, when writing data into the string unit SU1, the string unitsSU1, and the string unit SU2 that shares memory pillars MP with thestring unit SU1 are targeted for pre-verifying. As shown in FIGS. 3 and4, in this example, the string unit SU1 also shares the memory pillarsMP with the string unit SU0. However, as shown in FIG. 10, data iswritten in the string unit SU0 before writing data into the string unitSU1. Thus, the string unit SU0 does not require the pre-verifyingoperation for over-erased cell transistors. Targeted in the pre-verifyoperation are, for the string unit SU1, all the bit lines BL coupled tothe string unit SU1, and for the string unit SU2, the bit lines BL(4j+1), BL(4 j+2), and BL(4 j+3) coupled to the memory pillars MP sharedby the string unit SU1 and the string unit SU2.

Similarly, when writing data into the string unit SU2, according to theconfiguration of the string units SU illustrated in FIGS. 3 and 4 andthe table of the data writing order illustrated in FIG. 10, the stringunits SU2 and the string unit SU3 sharing memory pillars MP with thestring unit SU2 are targeted for pre-verifying. All the bit lines BL ofthe string unit SU2 are pre-verify targets, while the bit lines BL(4 j)coupled to the shared memory pillars are a pre-verify target for thestring unit SU3.

The pre-verify operations are executed in the same manner when writingdata into the string units SU3 to SU6. When writing data into the stringunit SU7, according to the data writing order shown in FIG. 10, thepre-verify and pre-program operations and the data write operation forthe adjacent string units SU6 and SU0 are already completed. Thus, onlythe string unit SU7 is targeted for pre-verifying.

As a result of the pre-verify operation, the bit lines BL correspondingto the over-erased cell transistors are selected so that the pre-programcan be executed for the over-erased cell transistors.

1.3 Effects According to First Embodiment

According to the first embodiment, reliability in the operation of thesemiconductor memory device can be enhanced. The effects are discussedin detail below.

With the method of writing data into the semiconductor memory deviceaccording to the present embodiment, a pre-verify operation is executedbefore data programming. In particular, the verify operation is executedon a memory cell transistor into which data is to be written (referredto as a selected cell) and a non-selected memory cell transistor(hereinafter referred as a backside cell transistor) that is arranged onthe same layer of the same memory pillar MP as the selected cell, asshown in FIGS. 6 and 7. Whether the threshold voltage value of thebackside cell transistor is smaller than or equal to the voltage Vcut1is thereby checked. When the threshold voltage value of the backsidecell transistor is smaller than or equal to the voltage Vcut1 (i.e. withthe backside cell transistor being an over-erased cell transistor), thepre-program operation is executed so that the threshold voltage value ofthe backside cell transistor is increased to a value larger than thevoltage Vcut1.

In this manner, in the program verify operation, the memory celltransistors MT0 to MT7 of the non-selected string unit SU can be in anoff state. Thus, in the program verify operation, the reliability of theverify result of the memory cell transistor MT based on the on/off statewhen the voltage Vvfy is applied to its control gate can be improved.

In the structure of the memory pillar MP illustrated in FIGS. 6 and 7,two memory cell transistors MT (a selected cell transistor and abackside cell transistor) that face each other across a memory pillar MPor are positioned on the opposite sides of the memory pillar MP sharethe semiconductor layer 31 of the memory pillar MP. Thus, when one ofthe two facing memory cell transistors MT is turned on, a cell currentflows through the turned-on memory cell transistor. In order to executethe program verify operation for the selected cell, the voltage Vcut1 isapplied to the selected cell transistor. If the selected sell transistoris properly programmed so that the threshold voltage value of theselected cell transistor is sufficiently increased, the cell currentdoes not flow through the selected cell transistor. However, the voltageVcut1 is also applied to the control gate of the backside celltransistor during the program verify operation. If the threshold valueof the backside cell transistor is smaller than or equal to the Vcut1(i.e. with the backside cell being an over-erased cell transistor), thecell current may still flow through the backside cell transistor evenafter the threshold voltage value of the selected cell is sufficientlyincreased, as a result of which the program verify operation ends infailure.

According to the present embodiment, the backside cell transistor cansecurely be in the off state during the verify operation so that thecell current can be prevented from flowing through this backside celltransistor. As a result, whether a cell current flows or not isdetermined in accordance with whether a selected cell is in the ON stateor OFF state when the voltage Vvfy is applied, or in other words, inaccordance with whether the threshold voltage value of the selected cellis smaller than or equal to the voltage Vvfy, or larger than the voltageVvfy. In this manner, the adverse influence of the backside celltransistor can be reduced.

In the example of FIG. 11, the pre-program operation is executed afterthe pre-verify operation. However, in case where no over-erased celltransistor is found as a result of steps S12 and S14, the pre-programoperation at steps S15 and S16 may be omitted.

2. Second Embodiment

A semiconductor memory device according to a second embodiment will beexplained. A method of writing data into a semiconductor memory devicewhich is different from the first embodiment is adopted in the secondembodiment. In particular, the absence of an over-erased cell transistoris determined after the pre-program operation is performed according tothe first embodiment, and then, the data program operation in step S17and subsequent steps is executed. In the following description, only thedifferences with respect to the first embodiment will be explained.

2.1 Data Write Method

FIG. 14 shows a flowchart of the write operation in a semiconductormemory device according to the present embodiment. As illustrated here,what is different from the first embodiment in FIG. 11 is that, afterpre-verifying at steps S11 to S14, pre-programming is executed at stepsS15 to S16 when a string unit including an over-erased cell transistoris found. In particular, whether or not the threshold value of thememory cell transistor MTi is higher than Vcut1 is determined at stepsS11 to S14, and pre-programming is executed at steps S15 to S16 if thepresence of any over-erased cell transistor is observed (YES at stepS30). Thereafter, whether or not the threshold value of the memory celltransistor MTi becomes higher than Vcut1 is checked again at steps S11to S14, and if the presence of any over-erased cell transistor is stillobserved (YES at step S30), steps S15 to S16 and S11 to S14 arerepeated. When an over-erased cell is no longer found (or when thenumber of over-erased cells falls below a predetermined number), theoperation proceeds to steps S17 to S21 to execute the data programoperation. Thus, the presence of any over-erased cell transistors arechecked again after the pre-program operation is performed at steps S15and S16.

FIG. 15 shows a timing chart of the transition of voltages of the signallines during the data write operation according to the secondembodiment, which corresponds to FIG. 12 illustrating the timing chartduring the data write operation according to the first embodiment. FIG.12 illustrates a situation in which an over-erased cell is found at stepS30 after the first pre-verify operation is performed. As illustrated inFIG. 15, after the pre-program operation is executed during times t2 tot3′, the pre-verify operation is again executed for the string unit SU0during times t3′ to t3″. This pre-verify operation is the same as thepre-verify operation executed during times t0 to t1. Thereafter, duringthe times t3″ to t3, the pre-verify operation is executed for each ofthe string units SU1 and SU7. This pre-verify operation is the same asthe pre-verify operation executed during times t1 to t2, and theoperation is independently executed for the string units SU1 and SU7. Inparticular, first, the pre-verify operation begins with selection of theselection gate line SGD1 while not selecting the other selection gatelines SGD to pre-verify the string unit SU1. Then, the pre-verifyoperation is executed for the string unit SU7 by selecting the selectiongate line SGD7, while not selecting the other selection gate lines SGD.

The memory cell transistors MT for which it is pre-verified that theyare no longer over-erased cell transistors after pre-programming atsteps S15 and S16 will not be subjected to the next pre-programming, andthe voltage VSG may be applied to the bit lines BL corresponding tothose memory cell transistors MT.

2.2 Effects According to Second Embodiment

According to the second embodiment, it is confirmed that the thresholdvoltages of memory cell transistors MTi in the erased state are higherthan the voltage Vcut1 by pre-verifying the memory cell transistors MTiafter pre-programming. After it is confirmed, the data program operationis executed. Thus, the reliability of the data write operation can beenhanced.

3. Third Embodiment

Next, a semiconductor memory device according to a third embodiment willbe discussed. In the third embodiment, a method of writing data into thesemiconductor memory device that is different from the first and secondembodiments is adopted. In particular, the program operation is executedfor memory cell transistors targeted for the data write operation beforethe pre-verify operation as discussed according to the first and secondembodiments. In the following explanation, only differences with respectto the first and second embodiments will be discussed.

3.1 Data Write Method

FIG. 16 shows a flowchart of the data write operation in a semiconductormemory device according to the third embodiment. As shown in thisflowchart, the write operation differs from FIG. 11 of the firstembodiment in that upon receipt of a write command at step S10, theprogram operation is executed for a string unit SU designated by theaddress that is received from the controller 200, before executing thepre-verify operation at steps S11 to S14. In detail, the sequencer 170selects the string unit SU designated by the address that is receivedfrom the controller 200 (step S40). Then, the sequencer 170 executes theprogram operation for the string unit SU selected at step S40. In thisoperation, the word line WLi designated by the controller 200 isselected, and a voltage VPGM is applied to the selected word line WLi asa program voltage (step S41). Then, the operations of step S11 andsubsequent steps as discussed in the first embodiment are executed.

FIG. 16 illustrates the data write operation in which the operation ofthe third embodiment is applied to the data write operation according tothe first embodiment as shown in FIG. 11. However, the operation of thethird embodiment may also be applied to the data write operation of thesecond embodiment. In this case, the operation of the third embodimentmay include performing steps S40 and S41 after step S10 and before stepS11 as shown in FIG. 14 while step S11 is executed after step S16.

FIG. 17 shows a timing chart of the voltages of the signal lines duringthe data write operation in a semiconductor memory device according tothe third embodiment, which corresponds to the timing chart according tothe first embodiment as shown in FIG. 12. As illustrated in FIG. 17, theprogram operation is executed during times t0′ to t0 before time to.This program operation is the same as the program operation executedduring times t3 and t4. In the program operation executed during timest0′ to t0 as shown in of FIG. 17, however, all the bit lines BL aretargeted for the write operation. Therefore, the sense amplifier 140applies, for example, 0V to all the bit lines BL.

3.2 Effects According to Third Embodiment

According to the third embodiment, the program operation is executed onthe selected memory cell transistors before the pre-verify operation isperformed. In this manner, the threshold values of the selected memorycell transistors can be set higher than the voltage Vcut1. In theexample shown in FIG. 17, a single program pulse is applied during thetimes t0′ to t0. However, multiple program pulses may be applied duringthe times t0′ to t0.

4. Fourth Embodiment

A semiconductor memory device according to the fourth embodiment will beexplained. In the fourth embodiment, a writing order different from thewriting order in the semiconductor memory device according to the firstembodiment is adopted so that the number of pre-verify operations andpre-program operations can be reduced. In the following explanation,only differences with respect to the first embodiment will be discussed.

4.1 Writing Order

FIG. 18 a data writing (selection) order for the string units SU0 to SU7and the word lines WL0 to WL7 when writing data into a certain block BLKof the semiconductor memory device.

In this example, as shown in FIG. 18, like FIG. 10 referred to in theexplanation of the first embodiment, a higher priority is assigned tothe selection of a word line WL than the selection of a string unit SU.However, the writing order of FIG. 18 differs from FIG. 10 in that ahigher priority is assigned to even string units SUe than odd stringunits SUo.

In particular, first, the word line WL0 of the string unit SU0 isselected, then the word line WL0 of the string unit SU2 is selected, andthen, similarly, the word lines WL0 of the string units SU4 and SU6 areselected. Thereafter, the word lines WL0 of the odd string units SUo areselected in turn. That is, the word line WL0 of the string unit SU1 isselected, the word line WL0 of the string unit SU3 is selected, and thenthe word lines WL0 of the string units SU5 and SU7 are selected in asimilar manner. Thereafter, the word line WL1 is selected. The selectionof the word line WL1 is performed in a similar manner, for each of thestring units SU0, SU2, SU4, SU6, SU1, SU3, SU5, and SU7 selected in thisorder. Finally in this sequence of selection of the string unit SU andthe word line WL for writing data, the word line WL7 of the string unitSU7 is selected.

4.2 Data Write Method

FIG. 19 shows a flowchart of the data write operation in thesemiconductor memory device according to the fourth embodiment. As shownin the flowchart of FIG. 19, the fourth embodiment differs from theflowchart of FIG. 11 according to the first embodiment in that, afterstep S10, the sequencer 170 determines whether a string unit SUdesignated by the address received from the controller 200 is an evenstring unit or an odd string unit. If the designated string unit SU isan even string unit (YES at step S51), the sequencer 170 executes theoperations in step S11 and subsequent steps, as explained in the firstembodiment. On the other hand, if it is an odd string unit (NO at stepS51), the sequencer 170 executes the data program operation in steps S17to S21 by skipping steps S11 to S16.

FIG. 20 shows string units SU targeted for the pre-verify andpre-program operations when writing data into the even string units SU0.SU2, SU4, and SU6. In the same manner as the first embodiment discussedwith reference to FIG. 13, “*1” in this table indicates that the bitlines BL(4 j) are pre-verified, “*2” indicates that the bit lines BL(4j+1), BL(4 j+2), and BL(4 j+3) are pre-verified. No mark indicates thatthe bit lines BL(4 j), BL(4 j+1), BL(4 j+2), and BL(4 j+3) arepre-verified.

When writing data into the string unit SU0, the string units SU0, SU1,and SU7 are targeted for the pre-verify operation. All the bit lines BLof the string unit SU0 are pre-verified; the bit lines BL(4 j) of thestring unit SU1 are pre-verified; and the bit lines BL(4 j+1), BL(4j+2), and BL(4 j+3) of the string unit SU7 are pre-verified, which islike the first embodiment as discussed with reference to FIG. 13.

When writing data into the string unit SU2, the string unit SU1, inaddition to the string units SU2 and SU3, is also pre-verified. For thestring unit SU1, the bit lines BL(4 j+1), BL(4 j+2), and BL(4 j+3) arepre-verified. The string unit SU1 should be pre-verified because, asshown in FIGS. 3 and 4, the string unit SU2 is positioned adjacent tothe string unit SU1 so that the string unit SU2 shares the memorypillars MP (i.e. MP1, MP8, MP9, MP17, MP24 and MP25) with the stringunit SU, and when writing to the string unit SU2, no writing orprogramming is yet to be performed for the memory cell transistors MT ofthe string unit SU1. In the example of FIGS. 3 and 4, the memory pillarsMP shared by the string units SU1 and SU2 are memory pillars MP1, MP8,MP9, MP17, MP24 and MP25, to which the bit lines BL1 to BL3 and BL5 toBL7 are coupled. Thus, for the string unit SU, the bit lines BL to BL3and BL5 to BL7 represented by BL(4 j+1), BL(4 j+2), and BL(4 j+3) arepre-verified.

Similarly, when writing data into the string unit SU4, the string unitsSU3, SU4 and SU are pre-verified. For the string unit SU3, the bit linesBL(4 j+1), BL(4 j+2) and BL(4 j+3) are pre-verified depending on thememory pillars MP shared with the string unit SU4. Furthermore, whenwriting data into the string unit SU6, the string units SU5, SU6, andSU7 are pre-verified. For the string unit SU5, the bit lines BL(4 j+1),BL(4 j+2), and BL(4 j+3) are pre-verified.

When writing data into the odd string units SU1, SU3, SU5, and SU7, thedata writing or programming for the even string units SU0, SU2, SU4, andSU6 has been completed as shown in FIG. 18. Thus, neither pre-verifyingnor pre-programming is required to write data into the odd string unitsSU1, SU3, SU5, and SU7.

A writing order is not limited to the writing order of FIG. 18. Data maybe written into the even string units SU0, SU2, SU4, and SU6 afterwriting data into the odd string units SU1, SU3, SU5, and SU7.

4.3 Effects According Fourth Embodiment

According to the fourth embodiment, by optimally setting the writingorder, the pre-verify operation and pre-program operation can beexecuted only either for the even string units or for the odd stringunits. Thus, the processing speed of the write operation can beincreased.

The determination at steps S50 and S51 of FIG. 19 may be performed bythe NAND flash memory 100. Whether the designated string unit is even orodd may also be designated by a command from the controller 200. For thecase of designating it by the command, a command sequence transmittedfrom the controller 200 to the NAND flash memory 100 is shown FIG. 21.

As shown in FIG. 21, the controller 200 first transmits lower page datato the NAND flash memory 100.

Such a command sequence includes:

<XXh><01h><80h><CA1><CA2><RA1><RA2><RA3><DATA><1Xh>

The “XXh” is a prefix command indicating that the NAND flash memory 100is instructed to execute a pre-verify operation and a pre-programoperation for an even string unit which is a data write target. The“01h” indicates that a lower page data is sent, and the “80h” is used todeclare that an address input will follow. After column address and rowaddress are transmitted over five cycles, the lower page data istransmitted. Finally, the “1Xh” command is transmitted. The “1Xh” is acommand for instructing the NAND flash memory 100 so that the insideregisters receive the transmitted data. Upon reception of this command“1Xh”, the NAND flash memory 100 goes into a busy state, and the lowerpage data is transferred to the sense amplifier 140. Thereafter, whenthe NAND flash memory 100 returns to a ready state, the controller 200transmits the middle page data to the NAND flash memory 100. Thiscommand sequence includes:

<02h><80h><CA1><CA2><RA1><RA2><RA3><DATA><1Xh>

The “02h” indicates that a middle page data is sent. When the “1Xh”command is transmitted to the NAND flash memory 100, the NAND flashmemory 100 goes into a busy state, and the middle page data istransferred to the sense amplifier 140. Thereafter, when the NAND flashmemory 100 returns to a ready state, the controller 200 transmits theupper page data to the NAND flash memory 100. This command sequenceincludes:

<03h><80h><CA1><CA2><RA1><RA2><RA3><DATA><10h>

The “03h” indicates that an upper page data is sent. When the “10h”command is transmitted to the NAND flash memory 100, the NAND flashmemory 100 goes into a busy state, and the upper page data istransferred to the sense amplifier 140. In the NAND flash memory 100,the transferred 3-page data is written into the memory cell array 110.That is, in response to reception of the “10h” command, the NAND flashmemory 100 goes from the ready state into the busy state, and thepre-verify operation and pre-program operation are executed for the evenstring unit designated by the address. Then, the data program operationis executed, and thereafter the NAND flash memory 100 returns from thebusy state to the ready state. On the other hand, when an odd stringunit is designated by an address as the data write target, after theNAND flash memory 100 goes into the busy state, the data programoperation is executed without executing the pre-verify and pre-programoperations. Thereafter, the NAND flash memory 100 returns to the readystate.

As discussed above, whether or not to execute the pre-verify andpre-program operations may be determined in accordance with the commandof the controller 200.

Furthermore, a writing order is not limited to the writing order shownin FIG. 18, and may be the writing order as indicated in FIG. 22. Asshown in FIG. 22, first, data may be written for the word lines WL0 toWL7 in each of the even string units SU0, SU2, SU4, and SU6, andthereafter, for the word lines WL0 to WL7 in each of the odd stringunits SU1, SU3, SU5, and SU7. The same effects can be achieved with thiswriting order.

5. Fifth Embodiment

Next, a semiconductor memory device according to the fifth embodimentwill be explained. According to the fifth embodiment, a two-stage datawriting scheme is applied to the semiconductor memory device of thefirst to fourth embodiments. Such a scheme is referred to as “2-stageprogramming”. The following explanation focuses only on differences ofthe fifth embodiment with respect to the first to fourth embodiments.

5.1 2—Stage Programming

First, the 2-stage programming according to the present embodiment willbe explained by citing two examples.

1st Example

FIG. 23 shows respective steps performed for writing data into memorycell transistors MT coupled to word lines WLi and WL(i+1) in the 2-stageprogramming according to 1st Example by illustrating the thresholdvoltage distributions of the memory cell transistors MT.

As shown in FIG. 23, at the first step, both the memory cell transistorsMT coupled to the word lines WLi and WL(i+1) are in the “Er” state. Inthis “Er” state, the word line WLi is selected to execute the programoperation for the memory cell transistor MT to be set to any of the “D”to “G” states. The verify voltage level used for this programming has amiddle level voltage Vfy_LM which is lower than the target thresholdvoltage value. As a result, as shown in the second step, the thresholdvoltage of the memory cell transistor MT coupled to the word line WLi isarranged in a middle-level distribution, which is referred to as an “LM”state. The “LM” state may range from the “D” state to the “G” state. Theprogram operation for the “LM” state is referred to as first stageprogramming.

Next, as shown in the third step, the word line WL(i+1) positionedadjacent to the word line WLi on its drain side is selected, and thefirst stage programming is executed for the memory cell transistor MTcoupled to the word line WL(i+1). Due to this first stage programmingfor the word line WL(i+1), the memory cell transistor MT coupled to theword line WLi is affected by the inter-cell interference effect, and itsthreshold voltage distribution is shifted toward the positive voltageside.

Thereafter, as shown in the fourth step, the word line WLi is selectedto write data. The verify voltage level is set to the target verifyvoltage level. In this program operation, the “A”, “B”, or “C” state canbe made from the “Er” state, and the “D”, “E”, “F”, or “G” state fromthe “LM” state. This program operation is referred to as second stageprogramming. Since the transition of the threshold voltage of the memorycell transistor MT in the second stage programming is smaller than forthe full sequence programming, the amount of the threshold voltage valueshift for the memory cell transistor MT in the fourth step can bereduced.

Next, as shown in the fifth step, the word line WL(i+1) is selected toexecute the second stage programming is executed for the memory celltransistor MT coupled to the word line WL(i+1). When executing thesecond stage programming for the word line WL(i+1), the first stageprogramming for the word line WL(i+2) is already completed.

Thus, as shown in the sixth step, in the 2-stage programming of the 1stExample, the inter-cell interference effect may be reduced.

2nd Example

The 2-stage programming according to 2nd Example is explained. FIG. 24shows respective steps performed for writing data into the memory celltransistors MT coupled to the word lines WLi and WL(i+1) in the 2-stageprogramming according to the 2nd Example by illustrating the thresholddistributions of the memory cell transistors MT.

As shown in FIG. 24, at the first step, both the memory cell transistorsMT coupled to the word lines WLi and WL(i+1) are in the “Er” state. Inthis state, the word line WLi is selected to write data into the memorycell transistor MT coupled to the word line WLi. The verify voltagesadopted here are set to the voltage levels VfyA′, VfyB′, VfyC′, . . . ,which are lower than the target verify voltage levels VfyA, VfyB, VfyC,. . . , respectively. As a result, as shown in the second step, thethreshold voltage of the memory cell transistor MT coupled to the wordline WLi is temporarily set in the foggy threshold voltage distributionof the “A”, “B”, “C”, . . . states. This is referred to as first stageprogramming of the 2nd Example.

Next, as indicated at the third step, the word line WL(i+1) is selectedto execute the first stage programming for the memory cell transistor MTcoupled to the word line WL(i+1) in a similar manner. With this firststage programming for the word line WL(i+1), the memory cell transistorMT coupled to the word line WLi is affected by the inter-cellinterference effect, and its threshold distribution is shifted towardthe positive voltage side.

Thereafter, as indicated in the fourth step, the word line WLi isselected to write data. The verify voltages adopted here are set totarget verify voltage levels VfyA, VfyB, VfyC, . . . . Since thethreshold voltage of the memory cell transistor MT coupled to the wordline WLi is programmed in the foggy threshold voltage distributioncorresponding to any of the “A” state, “B” state, “C” state, . . . , theamount of the threshold voltage shift at the fourth step can be reduced.This programming is referred to as second stage programming of the 2ndExample.

Next, as indicated in the fifth step, the word line WL(i+1) is selectedto execute the second stage programming for the memory cell transistorMT coupled to the word line WL(i+1). In the 2nd Example, at the time ofexecuting the second stage programming for the word line WL(i+1), thefirst stage programming for the word line WL(i+2) is already completed.

As a result, as indicated in the sixth step, in the 2-stage programmingof the 2nd Example, the inter-cell interference effect can be reduced.

5.2 Writing Order

FIG. 25 shows a writing order with respect to the string units SU0 toSU7 and the word lines WL0 to WL7 (i.e. a selection order thereof) whenwriting data into a certain block BLK. In FIG. 25, “1st” and “2nd”denote execution of the first and second stage programming,respectively. FIG. 25 shows the writing order for the first stageprogramming and the second stage programming.

As shown in FIG. 25, a higher priority is assigned to the selection of aword line WL than the selection of a string unit SU in this example, inthe same manner as the first embodiment illustrated in FIG. 10. Thisexample, however, differs from FIG. 10 in that, firstly, the first stageprogramming for the word line WL0 of the string units SU0 to SU7 isexecuted, and secondly, the first stage programming for the word line WLof the string units SU0 to SU7 is executed in a similar manner, andthereafter the second stage programming for the word line WL0 of thestring units SU0 to SU7 is executed.

In other words, with a certain word line WL selected, the string unitsSU0 to SU7 are selected in turn. The second stage programming for theword line WLi of a certain string unit SU is executed after the firststage programming for the word line WL(i+1) of this string unit SU iscompleted. This writing order as well as the various writing ordersdiscussed below is applied to all the 2-stage programming schemes.

5.3 Method of Writing Data

FIG. 26 shows a flowchart of the write operation according to the fifthembodiment. As shown in FIG. 26, steps S10 to S17 are performed in thesame manner as in the first embodiment discussed with reference to FIG.11. Thereafter, the first stage programming is executed for word lineWLi of the string units SU0 to SU7 (step S60). When the first stageprogramming is completed for the word lines WL(i+1) of the string unitsSU0 to SU7, the second stage programming is executed for the word lineWLi of the string units SU0 to SU7 (step S61). The detailed operationsof both the first stage programming and the second stage programming arethe same as the operations performed during times t3 to t5 as explainedin the first embodiment with reference to FIG. 12, except for the verifyvoltages adopted.

The data write operation is executed in the above-described manner. Asshown in FIG. 27, the pre-verify operation and the pre-program operationare executed in the first stage programming, but are not executed in thesecond stage programming. This scheme is illustrated in FIG. 27 whichshows the string units SU targeted for the pre-verify and pre-programoperations at the time of executing the first stage program and thesecond stage program when writing data into each of the string units SU0to SU7. In FIG. 27, like FIG. 13 of the first embodiment, “*1” indicatesthat the bit lines BL(4 j) are pre-verified, “*2” indicates that the bitlines BL(4 j+1), BL(4 j+2), and BL(4 j+3) are pre-verified. No markrepresents the bit lines BL(4 j), BL(4 j+1), BL(4 j+2), and BL(4 j+3)are pre-verified.

As shown in FIG. 27, the string units SU targeted for the pre-verifyoperation and the pre-program operation at the time of executing thefirst stage programming are the same as the string units SU targeted inthe first embodiment explained with reference to FIG. 13, for the samereason as the first embodiment. In contrast, at the time of executingthe second stage programming, neither pre-verify operation norpre-program operation is executed. This is because at the time of thesecond stage program, the first stage programming for the adjacentstring units SU that share the memory pillars MP with the write targetstring unit SU is already completed, and therefore the possibility ofthe backside cells on the shared memory pillars MP being an over-erasedcell is low.

5.4 Effects According to Fifth Embodiment

As discussed above, the write method explained in the first embodimentis applicable to the 2-stage programming. Alternatively, one of thesecond to fourth embodiments may be applied.

6. Sixth Embodiment

Next, a semiconductor memory device according to the sixth embodimentwill be explained. This embodiment relates to the modification examplesof the writing order as described in the fifth embodiment. The followingdiscussion will focus on differences of the sixth embodiment withrespect to the fifth embodiment.

6.1 1st Example

FIG. 28 shows another writing order with respect to the string units SU0to SU7 and the word lines WL0 to WL7 (i.e. another selection orderthereof) according to 1st Example.

In the order according to this example, the higher priority is assignedto the selection of word lines WL in the same manner as in the fifthembodiment, and for the string units SU, the higher priority is assignedto the selection of even string units SUe than odd string units SUo.

In the 1st Example, first, the string units SU0, SU2, SU4, and SU6 areselected for the word line WL0 to execute the first stage programming,and then the string units SU1, SU3, SU5, and SU7 are selected to executethe first stage programming. Next, the string units SU0, SU2, SU4, andSU6 are selected for the word line WL1 to execute the first stageprogramming, and then the string units SU1, SU3, SU5, and SU7 areselected to execute the first stage programming. Thereafter, the stringunits SU0, SU2, SU4, and SU6 are selected for the word line WL0 toexecute the second stage programming, and then the string units SU1,SU3, SU5, and SU7 are selected to execute the second stage programming.The same operations follow afterwards.

According to the order of the 1st Example, the pre-verify operation andthe pre-program operation are executed in the first stage program, in amanner similar to the operations shown in FIG. 20 for the fourthembodiment.

6.2 2nd Example

FIG. 29 another writing order with respect to the string units SU0 toSU7 and the word lines WL0 to WL7 (i.e. another selection order thereof)according to 2nd Example.

According to the order of the 2nd Example, after the first and secondstage programming for the even string units SUe are completed, the firstand second stage programming for the odd string units SUo are executed.

In particular, in the 2nd Example, first, the string units SU0, SU2,SU4, and SU6 are selected for the word line WL0 to execute the firststage programming, and then the string units SU0, SU2, SU4, and SU6 areselected for the word line WL1 to execute the first stage programming.Next, the string units SU0, SU2, SU4, and SU6 are selected for the wordline WL0 to execute the second stage programming, and then the stringunits SU0, SU2, SU4, and SU6 are selected for the word line WL2 toexecute the first stage programming. When the second programming for theword line WL7 of the string unit SU6 is completed, the first and secondstage programming for the odd string units SUo is initiated in the samemanner.

In the writing order of the 2nd Example, the pre-verify operation andthe pre-program operation in the first stage program are executed in amanner similar to the operations as illustrated in FIG. 20 for thefourth embodiment.

6.3 3rd Example

FIG. 30 is shows another writing order with respect to the string unitsSU0 to SU7 and the word lines WL0 to WL7 (i.e. another selection orderthereof) according to 3rd Example.

According to the order of the 3rd Example, the first stage programmingfor the respective word lines WL0 and WL1 of the string unit SU0 isexecuted, and then the first stage programming for the respective wordlines WL0 and WL1 of the string unit SU1 is executed. In a similarmanner, the first stage programming for the word lines WL0 and WL1 ofthe string units SU2 to SU7 is executed in turn.

Thereafter, the second stage programming for the word line WL0 of thestring unit SU0 is executed, and then the first stage programming forthe word line WL2 of the string unit SU0 is executed. Next, the secondstage programming for the word line WL0 of the string unit SU1 isexecuted, and then the first stage programming for the word line WL2 ofthe string unit SU1 is executed. The similar operations followafterwards.

According to the order of the 3rd Example, the pre-verify operation andthe pre-program operation in the first stage programming are executed inthe same manner as the operations as shown in FIG. 13 for the firstembodiment.

6.4 4th Example

FIG. 31 shows another writing order with respect to the string units SU0to SU7 and the word lines WL0 to WL7 (i.e. another selection orderthereof) according to 4th Example.

In the 4th Example, the writing order of the 3rd Example as shown inFIG. 30 is modified so that a higher priority is assigned to the evenstring units SUe than to the odd string units SUo when executing theprogram operation.

In particular, in the 4th Example, the first stage programming for therespective word lines WL0 and WL1 of the string unit SU0 is executed,and then the first stage programming for the respective word lines WL0and WL1 of the string unit SU2 is executed. Thereafter, the first stageprogramming for the respective word lines WL0 and WL1 of each of thestring units SU4 and SU6 is executed in the same manner.

Thereafter, the first stage programming for the respective word linesWL0 and WL1 of each of the odd string units SU1, SU3, SU5 and SU7 isexecuted in the same manner.

Then, the second stage programming for the word line WL0 of the stringunit SU0 is executed, and the first stage programming for the word lineWL2 of the string unit SU0 is executed. Thereafter, the second stageprogramming for the word line WL0 of the string unit SU2 is executed,and the first stage programming for the word line WL2 of the string unitSU2 is executed. The same operations are executed for the even stringunits SU4 and SU6, and then the same operations are executed on therespective word lines WL0 and WL2 of each of the odd string units SU1,SU3, SU5, and SU7. In the same manner, the programming is executedalternately for the even string units SUe and for the odd string unitsSUo.

According to the writing order of the 4th Example, the pre-verifyoperation and the pre-program operation are executed in the first stageprogramming, in the same manner as the operations as shown in FIG. 20for the fourth embodiment.

6.5 5th Example

FIG. 32 shows another writing order with respect to the string units SU0to SU7 and the word lines WL0 to WL7 (i.e. another selection orderthereof) according to 5th Example.

According to the 5th Example, in the same manner as the 4th Exampledescribed with reference to FIG. 31, a higher priority is assigned tothe even string units SUe than to the odd string units SUo whenexecuting the programming.

In particular, in the 4th Example, the first stage programming for therespective word lines WL0 and WL1 of the string unit SU0 is executed,and then the first stage programming for the respective word lines WL0and WL1 of the string unit SU2 is executed. Thereafter, the first stageprogramming for the respective word lines WL0 and WL1 of each of thestring units SU4 and SU6 is executed in the same manner.

Thereafter, the second stage programming for the word line WL0 of thestring unit SU0 is executed, and then the first stage programming forthe word line WL2 of the string unit SU0 is executed. Next, the secondstage programming for the word line WL0 of the string unit SU2 isexecuted, and then the first stage programming for the word line WL2 ofthe string unit SU2 is executed. Thereafter, similar operations areexecuted for each of the even string units SU4 and SU6, and then similaroperations are executed for the respective word lines WL1 to WL7 of eachof the even string units SU0, SU2, SU4, and SU6. After the first andsecond stage programming for the even string units SU0, SU2, SU4, andSU6 is completed, the first and second stage programming for the oddstring units SU, SU3, SU5 and SU7 is executed.

In the writing order according to the 5th Example, the pre-verifyoperation and the pre-program operation in the first stage programmingare executed in a manner similar to the operations as shown in FIG. 20for the fourth embodiment.

6.6 6th Example

FIG. 33 shows another writing order with respect to the string units SU0to SU7 and the word lines WL0 to WL7 (i.e. another selection orderthereof) according to 6th Example.

In this example, basically, after the first stage programming for theword line WLk (where k is a natural number between 2 and 7) of thestring unit SU0 is executed, the second stage programming for the wordline WL(k−1) of the same string unit SU0 is executed. The first andsecond stage programming in the above-mentioned order is repeated foreach of the string units SU1 to SU7.

6.7 7th Example

FIG. 34 shows another writing order with respect to the string units SU0to SU7 and the word lines WL0 to WL7 (i.e. another selection orderthereof) according to 7th Example.

In the 7th Example, like the 6th Example described with reference toFIG. 33, after the first stage programming for the word line WLk (wherek is a natural number between 2 and 7) of the string unit SU0 isexecuted, the second stage programming for the word line WL(k−1) of thesame string unit SU0 is executed. The first and second stage programmingin the above-described order is repeated for each of the even stringunits SU2, SU4, and SU6.

Thereafter, the same operation is repeated for each of the odd stringunits SU1, SU3, SU5, and SU7.

6.8 8th Example

FIG. 35 shows another writing order with respect to the string units SU0to SU7 and the word lines WL0 to WL7 (i.e. another selection orderthereof) according to 8th Example.

In the example, in accordance with the rule defining the writing orderof the 7th Example described with reference to FIG. 34, the first andsecond stage programming for each of the even string units SU0, SU2,SU4, and SU6 is executed. After the data writing is completed for all ofthe even string units SU0, SU2, SU4, and SU6, the first and second stageprogramming for each of the odd string units SU1, SU3, SU5, and SU7 isexecuted following the same rule.

6.9 Effects According to Sixth Embodiment

As discussed above, various writing orders may be adopted for the2-stage programming.

7. Seventh Embodiment

A semiconductor memory device according to the seventh embodiment willbe explained. Like the sixth embodiment, the seventh embodiment relatesto the modification examples of the writing order according to the fifthembodiment. In particular, the seventh embodiment is applicable to themethod of writing data according to the 2nd Example of the fifthembodiment as described above with reference to FIG. 24. The followingexplanation will focus only on differences of the seventh embodimentwith respect to the fifth and sixth embodiments.

7.1 1st Example

FIG. 36 shows a writing order with respect to the string units SU0 toSU7 and the word lines WL0 to WL7 (i.e. a selection order thereof)according to 1st Example of the seventh embodiment.

For the word lines WL2 to WL7, the writing order according to this 1stExample is the same as the writing order shown in FIG. 33 for the sixthembodiment. In this 1st Example, the first stage programming for theword line WL0 is executed in the order of the string units SU0 to SU7.Thereafter, for each of the string units SU0 to SU7, the first stageprogramming is executed for the word line WL1, and the second stageprogramming is executed for the word line WL0. The rest of theoperations are executed in the same way as the operations shown in FIG.33.

7.2 2nd Example

FIG. 37 shows another writing order with respect to the string units SU0to SU7 and the word lines WL0 to WL7 (i.e. another selection orderthereof) according to 2nd Example of the seventh embodiment.

For the word lines WL2 to WL7, the writing order of this 2nd Example isthe same as the writing order shown in FIG. 34 for the sixth embodiment.For the word lines WL0 and WL1, first, the first stage programming forthe word lines WL0 of the even string units SU0, SU2, SU4 and SU6 isexecuted, and then the first stage programming for the word lines WL0 ofthe odd string units SU1, SU3, SU5 and SU7 is executed. Thereafter, foreach of the even string units SU0, SU2, SU4, and SU6, the first stageprogramming for the word line WL1 is executed, and the second stageprogramming for the word line WL0 is executed. Then, the programoperations for the odd string units SU, SU3, SU5 and SU7 are similarlyexecuted. The rest of the operations are the same as the operations asshown in FIG. 34.

7.3 3rd Example

FIG. 38 shows another writing order with respect to the string units SU0to SU7 and the word lines WL0 to WL7 (i.e. another selection orderthereof) according to 3rd Example of the seventh embodiment.

For the word lines WL2 to WL7, the writing order of this 3rd Example isthe same as the writing order shown in FIG. 35 for the sixth embodiment.For the word lines WL0 and WL1, the first stage programming for the wordline WL0 of the even string units SU0, SU2, SU4 and SU6 is executed.Then, for each of the even string units SU0, SU2, SU4, and SU6, thefirst stage programming for the word line WL1 is executed, and thesecond stage programming for the word line WL0 is executed. Theoperations for the odd string units SU1, SU3, SU5 and SU7 are similarlyexecuted. Other operations are the same as the operations shown in FIG.35.

7.4 Effects According to Seventh Embodiment

As discussed above, various writing orders can be adopted for the methodof writing data according to Example 2. Furthermore, with the writingorder according to the seventh embodiment, the second stage programmingcan be prioritized in comparison to the writing order as shown in FIG.25 in which the first stage programming is executed simply in the orderof the string units SU, and thereafter the second stage programming isexecuted. This reduces the length of the time during which thecontroller 200 holds the write data, thereby reducing the buffer memorycapacity of the controller.

8. Other Modification Examples

As described above, the semiconductor memory device according to theabove-described embodiments includes a first memory cell (e.g. SU0)capable of holding data, a second memory cell (e.g. SU1 or SU7) capableof holding data, a first word line coupled to the first memory cell, asecond word line coupled to the second memory cell, and a first bit linecapable of being electrically coupled to both the first memory cell andthe second memory cell. The first memory cell shares a semiconductorlayer with the second memory cell, and faces the second memory cellacross the semiconductor layer. To write data into the first memorycell, the semiconductor memory device executes a first operation, asecond operation, and a third operation. In the first operation, a firstvoltage (Vcut1) is applied to the second word line (pre-verifying). Inthe second operation, a second voltage (VPGM_L) higher than the firstvoltage is applied to the second word line after the first operation(pre-programming). In the third operation, after the second operation, athird voltage (VPGM) higher than the second voltage (VPGM_L) is appliedto the first word line, and a fourth voltage (VPASS) lower than both thesecond voltage (VPGM_L) and the third voltage (VPGM) is applied to thesecond word line (data programming).

With the above-described embodiments, the reliability of writing datainto the semiconductor memory device can be enhanced. Theabove-explained embodiments are presented by way of example, and variousmodifications may be applied. For example, according to theabove-described embodiments, each memory cell transistor MT holdsthree-bit data. Instead, two-bit data or four-or-more-bit data may beheld. Furthermore, various writing orders are introduced for theembodiments. However, the writing order is not limited to theabove-described orders, and the order may be changed. In addition, adummy transistor may be provided between the selection transistors ST1and ST2 and the memory cell transistors MT in the NAND strings 50. Thedummy transistor serves as a current path. Thus, when the correspondingstring unit SU is selected, the dummy transistor is turned on. The orderof the steps of the flowcharts may be changed.

Note that the semiconductor memory device according to each embodimentconcerning the present invention may have the following configuration:

(1) When the memory cell holds 2-bit data (“Er”, “A”, “B”, and “C”), thevoltage applied to the selected word line in the reading operation of Alevel may range from, for example, 0V to 0.55V. However, the presentembodiments are not limited to this, and the voltage may be set withinany one of the ranges of 0.1V to 0.24V, 0.21V to 0.31V, 0.31V to 0.4V,0.4V to 0.5V, and 0.5V to 0,55V.

The voltage applied to the selected word line in the reading operationof B level may range from, for example, 1.5V to 2.3V. However, thevoltage is not limited to this and may be set within any one of theranges of 1.65V to 1.8V, 1.8V to 1.95V, 1.95V to 2.1V, and 2.1V to 2.3V.

The voltage applied to the selected word line in the reading operationof C level may range from, for example, 3.0V to 4.0V. However, thevoltage is not limited to this and may be set within any one of theranges of 3.0V to 3.2V, 3.2V to 3.4V, 3.4V to 3.5V, 3.5V to 3.6V, and3.6V to 4.0V.

A time (tR) of the reading operation may be set within the range of, forexample, 25 to 38 μs, 38 to 70 μs, or 70 to 80 μs.

(2) A write operation may include a program operation and a verifyoperation. In the write operation, the voltage first applied to theselected word line in the program operation may range from, for example,13.7V to 14.3V. The voltage is not limited to this, and may be setwithin any one of the ranges of, for example, 13.7V to 14.0V and 14.0Vto 14.6V.

The voltage first applied to the selected word line when write-accessingan odd word line may be different from the voltage first applied to theselected word line when write-accessing an even word line.

If the program operation is ISPP (Incremental Step Pulse Program), thevoltage of step-up may be, for example, 0.5V.

The voltage applied to a non-selected word line may be set within therange of, for example, 6.0V to 7.3V. However, the voltage is not limitedto this, and may be set within the range of, for example, 7.3V to 8.4 orset to 6.0V or less.

The pass voltage to be applied may be changed depending on whether thenon-selected word line is an odd word line or an even word line.

A time (tProg) of the write operation may be set within the range of,for example, 1,700 μs to 1,800 μs, 1,800 μs to 1,900 μs, or 1,900 μs to2000 μs.

(3) In an erasing operation, the voltage first applied to the well,which may be formed in the upper portion of the semiconductor substrateand above which the memory cell may be arranged, may be set within therange of, for example, 12V to 13.6V. However, the voltage is not limitedto this, and may be set within the range of, for example, 13.6V to14.8V, 14.8V to 19.0V, 19.0V to 19.8V, or 19.8V to 21V.

A time (tErase) of the erasing operation may be set within the range of,for example, 3,000 μs to 4,000 μs, 4,000 μs to 5,000 μs, or 4,000 μs to9,000 μs.

(4) In the structure of a memory cell, a charge accumulation layer maybe arranged on a 4 to 10 nm thick tunnel insulation film. The chargeaccumulation layer may have a stacked structure of a 2 to3-nanometer-thick insulation film of SiN or SiON and 3 to8-nanometer-thick polysilicon. A metal such as Ru may be added to thepolysilicon. An insulation film is provided on the charge accumulationlayer. The insulation film may include a 4 to 10-nanometer-thick siliconoxide film sandwiched between a 3 to 10-nanometer-thick thick lowerhigh-k film and a 3 to 10-nanometer-thick upper high-k film. As thehigh-k film, HfO or the like may be usable. The silicon oxide film maybe thicker than the high-k film. A 30 to 70-nanometer-thick controlelectrode may be formed on a 3 to 10-nanometer-thick work functionadjusting material on the insulation film. Here, the work functionadjusting material may be a metal oxide film such as TaO or a metalnitride film such as TaN. As the control electrode, W or the like isusable.

An air gap may be formed between the memory cells.

In the above embodiments, a NAND flash memory has been exemplified asthe semiconductor storage device. However, the embodiments may beapplicable not only to the NAND flash memory but also to other generalsemiconductor memories, and also applicable to various kinds of storagedevices other than the semiconductor memories. In the flowchartsdescribed in the above embodiments, the order of processes may bechanged as long as it is possible.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A memory system comprising: a semiconductormemory device including a first memory cell, a second memory cell, afirst word line, a second word line and a first bit line, the firstmemory cell being capable of holding data, the second memory cell beingcapable of holding data, the first word line being coupled to the firstmemory cell, the second word line being coupled to the second memorycell, the first bit line being capable of being electrically coupled toboth the first memory cell and the second memory cell, the first memorycell sharing a semiconductor layer with the second memory cell; and acontroller configured to control the semiconductor memory device,wherein the controller is configured to send a write command, inresponse to the write command, the semiconductor memory device isconfigured to execute a first operation, a second operation afterexecuting the first operation, and a third operation after executing thesecond operation, the first operation includes applying a first voltageto the second word line, the second operation includes applying a secondvoltage higher than the first voltage to the second word line, and thethird operation includes applying a third voltage higher than the secondvoltage to the first word line, and applying a fourth voltage lower thanboth the second voltage and the third voltage to the second word line.2. The memory system according to claim 1, wherein the first voltage isa voltage being applied to the second word line when reading data fromthe first memory cell, and the fourth voltage is a voltage for turningon a memory cell included in the semiconductor memory device.
 3. Thememory system according to claim 1, wherein the first operation is forverifying the second memory cell, the second operation includes a firstprogram operation to increase a threshold value of the second memorycell based on a result of the first operation, and the third operationincludes a second program operation to write data being specified by thewrite command, into the first memory cell.
 4. The memory systemaccording to claim 3, wherein the semiconductor memory device isconfigured to execute a fourth operation after executing the thirdoperation, the second program operation includes executing a first writeoperation using a first program verify voltage, the fourth operationincludes a third program operation to write the specified data into thefirst memory cell, the third program operation including executing asecond write operation after the first write operation, the second writeoperation uses a second program verify voltage higher than the firstprogram verify voltage, and the second write operation in the fourthoperation is executed without executing the first operation and thesecond operation.
 5. The memory system according to claim 3, wherein thesecond operation includes an operation of programming a memory cellwhich is determined in the first operation that a threshold value islower than or equal to the first voltage.
 6. The memory system accordingto claim 1, wherein the semiconductor memory device is configured toexecute a fourth operation after executing the second operation, thefourth operation includes applying the first voltage to the second wordline, and the third operation is executed after the fourth operation. 7.The memory system according to claim 6, wherein the first operation isfor verifying the second memory cell, the second operation includes afirst program operation to increase a threshold value of the secondmemory cell based on a result of the first operation, the fourthoperation includes an operation to verify the second memory cell toconfirm a result of the second operation, and the third operationincludes a second program operation to write data being specified by thewrite command, into the first memory cell.
 8. The memory systemaccording to claim 3, wherein the semiconductor memory device isconfigured to execute a fourth operation after executing the thirdoperation, the second program operation includes executing a first writeoperation using a first program verify voltage, and the fourth operationincludes a third program operation to write the specified data into thefirst memory cell, the third program operation includes executing asecond write operation after executing the first write operation, thesecond write operation uses a second program verify voltage higher thanthe first program verify voltage, and the second write operation in thefourth operation is executed without executing the first operation andthe second operation.
 9. The memory system according to claim 8, whereinthe second operation includes an operation of programming a memory cellwhich is determined in the first operation that a threshold value islower than or equal to the first voltage.
 10. The memory systemaccording to claim 1, wherein the semiconductor memory device isconfigured to execute a fourth operation before executing the firstoperation, and the fourth operation includes applying the third voltageto the second word line.
 11. The memory system according to claim 10,wherein the fourth operation includes a third program operation toincrease a threshold value of the second memory cell, the firstoperation includes an operation to verify the second memory cell, thesecond operation includes a first program operation to increase athreshold value of the second memory cell based on a result of the firstoperation, and the third operation includes a second program operationto write data being specified by the write command, into the firstmemory cell.
 12. The memory system according to claim 11, wherein thesemiconductor memory device is configured to execute a fifth operationafter executing the third operation, the second program operationincludes executing a first write operation using a first program verifyvoltage; and the fifth operation includes a fourth program operation towrite the specified data into the first memory cell, the fourth programoperation including executing a second write operation after executingthe first write operation, the second write operation uses a secondprogram verify voltage higher than the first program verify voltage, andthe second write operation in the fourth operation is executed withoutexecuting the first operation and the second operation.
 13. The memorysystem according to claim 11, wherein the second operation includes anoperation of programming a memory cell which is determined in the firstoperation that a threshold value is lower than or equal to the firstvoltage.
 14. The memory system according to claim 1, wherein the firstvoltage is a negative voltage.
 15. A memory system comprising: asemiconductor memory device including a first memory cell, a secondmemory cell, a first word line, a second word line and a first bit line,the first memory cell being capable of holding data, the second memorycell being capable of holding data, the first word line being coupled tothe first memory cell, the second word line being coupled to the secondmemory cell, the first bit line being capable of being electricallycoupled to both the first memory cell and the second memory cell, thefirst memory cell sharing a semiconductor layer with the second memorycell; and a controller configured to control the semiconductor memorydevice, wherein the controller is configured to send a first writecommand specifying the first memory cell, in response to the first writecommand, the semiconductor memory device is configured to execute afirst operation, a second operation after executing the first operation,and a third operation after executing the second operation, the firstoperation includes applying a first voltage to a word line correspondingto a non-selected memory cell, the second operation includes applying asecond voltage higher than the first voltage to the word linecorresponding to the non-selected memory cell, the third operationincludes applying a third voltage higher than the second voltage to theword line corresponding to the non-selected memory cell and applying afourth voltage lower than both the third voltage and the second voltageto a word line corresponding to a selected memory cell, the controlleris configured to send a second write command specifying the secondmemory cell, and in response to the second write command, thesemiconductor memory device is configured to execute the third operationwithout executing the first and second operations executed during aperiod between the semiconductor memory device entering to a busy stateand returning to a ready state.
 16. The memory system according to claim15, wherein, the first voltage is a voltage being applied to the secondword line when data is read from the first memory cell, the fourthvoltage is a voltage for turning on a memory cell, and the first voltageis a negative voltage.
 17. The memory system according to claim 15,wherein the semiconductor memory device includes a third memory cellcapable of holding data and a third word line coupled to the thirdmemory cell, the second word line is positioned between the first wordline and the third word line, and the semiconductor memory device isconfigured to write data into the second memory cell after writing datainto the first memory cell and the third memory cell.
 18. The memorysystem according to claim 15, wherein the first operation is executedfor verifying the non-selected memory cell, the second operationincludes a first program operation to increase a threshold value of thenon-selected memory cell based on a result of the first operation, andthe third operation includes a second program operation to write databeing specified by a write command specifying the selected memory cell,into the selected memory cell.
 19. The memory system according to claim18, wherein the semiconductor memory device is configured to execute afourth operation after executing the third operation, the second programoperation includes executing a first write operation using a firstprogram verify voltage, the fourth operation includes a third programoperation to write the specified data into the selected memory cell, thethird program operation includes executing a second write operationafter executing the first write operation, the second write operationuses a second program verify voltage higher than the first programverify voltage, and the second write operation in the fourth operationis executed without executing the first operation and the secondoperation.
 20. The memory system according to claim 18, wherein thesecond operation includes an operation of programming a memory cellwhich is determined in the first operation that a threshold value islower than or equal to the first voltage.